February 2014 

NASA/TM–2014-218174 

Bi-Metallic Composite Structures With 

Designed Internal Residual Stress Field 

Craig A. Brice 

Langley Research Center, Hampton, Virginia 
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA scientific and technical information (STI) 

program plays a key part in helping NASA maintain 

this important role. 

The NASA STI program operates under the 

auspices of the Agency Chief Information Officer. 

It collects, organizes, provides for archiving, and 

disseminates NASA’s STI. The NASA STI 

program provides access to the NASA Aeronautics 

and Space Database and its public interface, the 

NASA Technical Report Server, thus providing one 

of the largest collections of aeronautical and space 

science STI in the world. Results are published in 

both non-NASA channels and by NASA in the 

NASA STI Report Series, which includes the 

following report types: 

 TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

Programs and include extensive data or 

theoretical analysis. Includes compilations of 

significant scientific and technical data and 

information deemed to be of continuing 

reference value. NASA counterpart of peerreviewed formal professional papers, but 

having less stringent limitations on manuscript 

length and extent of graphic presentations. 

 TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or of 

specialized interest, e.g., quick release reports, 

working papers, and bibliographies that contain 

minimal annotation. Does not contain extensive 

analysis. 

 CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees. 

 CONFERENCE PUBLICATION. 

Collected papers from scientific and 

technical conferences, symposia, seminars, 

or other meetings sponsored or cosponsored by NASA. 

 SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, 

often concerned with subjects having 

substantial public interest. 

 TECHNICAL TRANSLATION. 

English-language translations of foreign 

scientific and technical material pertinent to 

NASA’s mission. 

Specialized services also include organizing 

and publishing research results, distributing 

specialized research announcements and feeds, 

providing information desk and personal search 

support, and enabling data exchange services. 

For more information about the NASA STI 

program, see the following: 

 Access the NASA STI program home page 

at http://www.sti.nasa.gov

 E-mail your question to help@sti.nasa.gov

 Fax your question to the NASA STI 

Information Desk at 443-757-5803 

 Phone the NASA STI Information Desk at 

443-757-5802 

 Write to: 

 STI Information Desk 

 NASA Center for AeroSpace Information 

 7115 Standard Drive 

 Hanover, MD 21076-1320
National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23681-2199 

 

February 2014 

NASA/TM–2014-218174 

Bi-Metallic Composite Structures With 

Designed Internal Residual Stress Field 

Craig A. Brice 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Acknowledgments 

The author gratefully acknowledges the contribution of the following individuals: 

Francisco Medina, Jorge Mireles, and Sara Gaytan at the W.M. Keck Center for 3D 

Innovation at the University of Texas at El Paso for their assistance in the design and 

fabrication of the EBM titanium components. Peter Messick, Joel Alexa, and Harold 

Claytor at NASA Langley Research Center for their assistance in preparation, processing, 

and analysis of the consolidated samples. Eric Burke at NASA Langley Research Center 

for his assistance in non-destructive evaluation and characterization. 

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an 

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics 

and Space Administration. 
1

 Begin your report below this instruction. Press F1 or Help for more. 

Abstract

Shape memory alloys (SMA) have a unique ability to recover small amounts of 

plastic strain through a temperature induced phase change. For these materials, 

mechanical displacement can be accomplished by heating the structure to induce a 

phase change, through which some of the plastic strain previously introduced to the 

structure can be reversed. This paper introduces a concept whereby an SMA phase is 

incorporated into a conventional alloy matrix in a co-continuous reticulated arrangement

forming a bi-metallic composite structure. Through memory activation of the 

mechanically constrained SMA phase, a controlled residual stress field is developed in 

the interior of the structure. The presented experimental data show that the memory 

activation of the SMA composite component significantly changes the residual stress 

distribution in the overall structure. Designing the structural arrangement of the two 

phases to produce a controlled residual stress field could be used to create structures 

that have much improved durability and damage tolerance properties.

Introduction

The shape memory effect has been well documented in the scientific literature [1]. 

Of particular interest for this work is the one-way shape memory effect in which the alloy 

is shape set at high temperature (e.g. 500°C), cooled and subsequently deformed 

plastically, then memory activated at low temperature (e.g. 100°C) in order to recover 

the initial product shape. The transformation between the high temperature austenite 

phase and the low temperature martensite phase is the mechanism by which this 

memory effect occurs. Austenite is the high temperature phase; upon cooling to room 

temperature, a phase change to martensite occurs. Crystallographically, this martensite 

phase transformation occurs by twinning. Untwinning of the martensite structure 

accommodates subsequent mechanical deformation. Finally, heat treatment above the 

austenite transformation temperature recovers the original crystal structure and the 

original shape of the part.

There are a wide variety of alloy systems that exhibit the shape memory effect [2]. 

One of the most prominent is a binary nickel titanium alloy known as Nitinol that was 

developed at the Naval Ordinance Laboratory in the 1960s [3]. The composition of 

Nitinol alloys is approximately 55 weight percent nickel and 45 weight percent titanium. 

Small variations in the ratio of nickel to titanium affect the phase transformation 

temperatures that govern the memory effect. The start of the martensite transformation 

can occur from well below room temperature (in this case, deformation at room 

temperature is recovered as soon as the applied load is removed—a condition termed 

“superelastic”) all the way up to about 70°C. For the application described in this paper, 

a high temperature composition is necessary to avoid premature transformation due to 

frictional heating during the deformation step in the processing sequence.

Shape memory alloys have been extensively researched for actuation applications in 

aero structures [4,5]. It has been well established that these alloys can exert a 

tremendous amount of force upon memory activation [6]. This paper describes a 

processing route for creating a metal matrix laminate composite with discrete SMA 

ligaments that will counteract any applied load and reduce the overall net effective load. 

A similar concept is routinely used in the construction industry. The tensile load carrying 

capacity of concrete (a notoriously brittle material in tension) is improved by poststressing the material with steel wires [7]. These wires pass through a conduit in the 
2

concrete and are elastically strained and locked into place once the concrete cures. The 

contraction of the steel wires is constrained by the concrete, resulting in constant 

residual compression that mitigates the propagation of cracks due to tensile loads. The 

proposed concept aims to take this well-proven civil engineering system and apply it to

aero structural designs with advanced material combinations.

The enabling feature of this concept is the application of additive manufacturing (AM) 

methods for the creation of the non-SMA component. Additive manufacturing is used to 

create the matrix component using a standard titanium alloy, Ti-6Al-4V. This matrix 

resembles an open-cell foam structure and enables the incorporation of a second phase 

into the structure. The voids in the matrix structure are filled with Nitinol SMA powder,

hot-consolidated and shape-set into a fully dense arrangement. The composite structure 

is then mechanically worked to introduce recoverable plastic strain into both the matrix 

and the SMA. Upon memory activation heat treatment, the SMA will attempt to revert to 

its original shape-set dimensions (i.e. contract) but will be constrained by the titanium 

alloy matrix. This will result in the development of a static residual stress field within the 

structure. As cracks generally nucleate at a free surface, the compressive field in front 

of the crack tip as it grows into the structure will inhibit propagation. The stress intensity 

factor (K) is related to the local stress field (σ) and crack length (a) by K = σ(πa)

1⁄2. 

Careful design of the structure and the resultant internal stress field should allow the 

stress intensity factor to remain below the threshold value, thus inhibiting any cracks 

from growing. Additionally, by the nature of the interlocking arrangement of the 

SMA/alloy composite core, any cracks initiating in this area will be contained and 

isolated from the load-bearing structural layers.

Experimental Procedure

An Arcam electron beam melting (EBM) AM machine at the W.M. Keck Center for 3D 

Innovation at the University of Texas at El Paso was used to make the matrix alloy net 

structure scaffolding coupons. Detailed description of the EBM process are presented 

elsewhere [8]. Two different designs were constructed; one using a periodic net 

structure and one using a foamy random structure. The periodic structure was designed 

using a 3D computer-aided design (CAD) modeling program. The foam structure was 

reverse-engineered using X-ray computed tomography from a piece of cast aluminum 

foam. Figure 1 shows cutaway sections of the CAD models, including the periodic 

structure unit cell and the actual coupons fabricated in Ti-6Al-4V using the Arcam EBM 

process. The coupons were 1.18 inches wide by 3.90 inches long by 0.48 inch thick. 

The net structure area was approximately 0.60 inch wide by 2.35 inches long by 0.20

inch thick.

The open cell net structure area of the AM fabricated coupons was filled with nickeltitanium shape memory alloy powder. The physical properties of the SMA powder are 

given in Table 1. The powder was poured into the void area in the net structure 

scaffolding and vibrated for a period of 15 minutes such that the powder could achieve 

peak tap density. A piece of wrought Ti-6Al-4V alloy plate (1.17 inches wide by 3.90 

inches long by 0.22 inch thick) was then used to cover the top surface of the coupon and 

the entire structure was vacuum hot pressed at 930°C for 4 hours at 1,000 psi. 

One of the hot pressed samples was examined via X-radiography in order to 

determine if the filling and consolidation procedure yielded a void-free structure (>99% 

density). Figure 2 shows the radiographic results and confirms >99% density. 

The samples were machined using traditional milling techniques to achieve a final 

sample width of 0.61 inch and a final thickness of 0.53 inch. This machining operation 

removed the excess matrix material from the outer edges leaving a two-dimensional 
3

laminate of Ti-6Al-4V EBM matrix, Ti-6Al-4V–SMA composite, and Ti-6Al-4V wrought 

plate. Figure 3 shows a cross-section of the initial coupon configuration with cut lines to 

indicate the final cross-sectional geometry. 

A shape-set heat treatment was performed at 500°C for 15 minutes, followed by 

furnace cooling. The samples where then cold rolled on a two-high laboratory rolling mill 

using a multi-pass procedure to an approximate overall reduction of 4%. The samples 

were monitored between passes to ensure that the overall temperature stayed below the 

austenite start temperature of 68°C. After the rolling operation, one sample from each of 

the designs was subjected to a memory activation heat treatment at 115°C for 15

minutes.

Three of the samples were sent to Hill Engineering, LLC, in Rancho Cordova, 

California, for cut compliance residual stress testing: two of the periodic mesh samples 

(one as-rolled as a baseline control, and one memory activation heat treated) and one of 

the foamy mesh samples in the memory activation heat treated condition. Details of the 

cut compliance residual stress testing methodology are presented elsewhere [9]. Cut 

compliance testing was performed at the approximate mid-point of the sample and the 

residual stress on the transverse plane was determined using strain gages. 

Results and Discussion

The visual comparison of the cut compliance tested samples is shown in Figure 4. 

Figure 4a shows the periodic net structure sample that has been cold rolled to 4% 

reduction but without the memory activation heat treatment. For cut compliance testing, 

wire electro-discharge machining (EDM) is used to make the cut through the sample

beginning at the top surface (relative to the image) and progressing down through the 

sample. The cut is visible in the sample and the kerf gap at the beginning of the cut is 

0.017 inch wide. Also note that the sample has slight concave curvature; this is due to

the cold rolling procedure and the elastic/plastic mismatch between the matrix Ti-6Al-4V 

alloy and the Ni-Ti shape memory alloy. Figure 4b shows the same configuration but 

with the addition of the memory activation heat treatment. The top of the EDM kerf has 

opened further (0.033 inch) due to stress relief as material is removed during the cutting 

process. A longitudinal crack between the bottom surface of the composite net structure 

area and the Ti-6Al-4V cover plate is also evident, as indicated by the arrow in Figure 

4b. These visual indications show that significant residual stress was introduced into the 

parts that have the SMA activated, and these stresses were relieved through the EDM 

cutting operation. The sample shown in Figure 4c is the foamy net structure that has 

been memory activated. This sample shows a much greater release of internal energy 

as evident by the significant kerf opening on the top of the sample (0.076 inch) and the 

long longitudinal crack at the net structure/cover plate interface. Also note that the 

release of residual stress in this sample has turned it from concave to convex.

The test data from the cut compliance testing is shown in Figure 5. The depth 

indicated on the abscissa is relative to the surface of the specimen where the EDM cut 

began. The interface between the solid Ti-6Al-4V matrix and the composite Ti-6Al-4V–

SMA area occurs at around 0.22 inch. The data set labeled “Periodic (control)” is from 

the sample with the periodic net structure that was cold rolled but not subjected to the 

memory activation heat treatment. The data indicates that the rolling process itself has 

introduced a residual stress field in the part. The difference in elastic modulus (16,000 

ksi for Ti-6Al-4V versus 5,800 ksi for Ni-Ti) contributes to this residual stress field in the 

as-rolled condition. The data set labeled “Periodic (memory)” is from the coupon with 

the same internal configuration as the control with the addition of the memory activation 

heat treatment after the rolling operation. The memory activation heat treatment 

changes the residual stress near the surface from +35 ksi to -10 ksi demonstrating a 
4

clear effect from the constrained SMA component. The data set labeled “Foam 

(memory)” is from the foamy net structure, and the data show that the residual stress 

profile is significantly different compared to the periodic net structure. In this case, the 

near-surface residual stress is +90 ksi, approximately 75% of the yield stress of Ti-6Al4V. These data demonstrate two key points: 1) the memory activated, fully constrained

SMA material imparts an internal stress to the matrix material, and 2) the structural 

configuration of the Ti-6Al-4V–SMA composite area can change the character and 

magnitude of the stress field generated. These results demonstrate the validity of the 

concept and suggest that a controlled residual stress field could be designed into a 

structure using bi-metallic composite configurations.

Summary

A bi-metallic composite structure was fabricated using AM and convetional powder 

metallurgy approaches. The AM process enabled the creation of titanium alloy open cell 

net structures that were subsequently filled with shape memory alloy powder and hot 

consolidated. Two configurations were analyzed: one with a periodic net structure and 

one with a foamy net structure. The fully dense, consolidated coupons were plastically 

strained using cold rolling and then subjected to a memory activation heat treatment. 

The interlocking nature of the two discrete phases constrained the memory effect in the 

SMA and induced a stress field into the matrix material. The two structural 

configurations showed different residual stress profiles indicating a correlation between 

stress state and net structure configuration. Further optimization of the net structure 

configuration should allow for the creation of customized residual stress profiles intended 

to improve specific properties of the overall structure.

References

1. K. Otsuka, C.M. Wayman, eds. Shape Memory Materials. Cambridge University 

Press: Cambridge, UK; 1998.

2. Z.G. Wei, R. Sandström. J. Mater. Sci. 1998; 33: 3743-3762.

3. W.J. Buehler, J.W. Gilfrich, R.C. Wiley. J. Appl. Phys. 1963; 34: 1473.

4. R. Chandra. Smart Mater. Struct. 2001; 10: 1018-1024.

5. D.J. Hartl and D.C. Lagoudas. Proc. Inst. Mech Engr, Part G: J. Aero Engr. 2007; 

221(4): 535-552.

6. D. Mavroidis, C. Pfeiffer, M. Mosley. Automation, Miniature Robotics and Sensors for 

Non-Destructive Testing and Evaluation. Y. Bar-Cohen ed. American Society for 

Nondestructive Testing: Columbus, OH; 1999.

7. A.H. Nilson. Design of Prestressed Concrete. J. Wiley & Sons: Hoboken, NJ; 1978.

8. I. Gibson, D.W. Rosen, B. Stucker. Additive Manufacturing Technologies: Rapid 

Prototyping to Direct Digital Manufacturing. Springer: New York; 2009.

9. H.J. Schindler. Advances in Fatigue Crack Closure Measurement and Analysis, 

ASTM STP 1343. R.C. McClung, J.C. Newman, Jr. eds. American Society for 

Testing and Materials: West Conshohocken, PA; 1998.
5

Table 1: Physical properties of nickel-titanium shape memory alloy powder.
6

Figure 1: Test coupon designs (top) showing section cut through net structure area. 

Periodic design unit cell shown in inset, upper left. Actual as-fabricated test coupons 

made via electron beam melting shown on bottom.
7

Figure 2: X-radiograph of as-consolidated periodic net shape structure. Red circles 

indicate isolated porosity.
8

Figure 3: Schematic of the cross-section configuration of the coupons. The dashed 

lines show approximately how much material was machined away in order to prepare 

the coupons for cut compliance testing. Also note the arrow showing which direction 

the cut was made with respect to the coupon orientation.
9

Figure 4: Side view photograph of as-tested cut compliance coupons: a) periodic 

mesh, cold rolled to 4% reduction, no memory activation heat treatment, b) periodic 

mesh, cold rolled to 4%, memory activation heat treatment, c) foam mesh, cold rolled 

4%, memory activation heat treatment. For clarity, the Ti-6Al-4V–SMA composite 

area is highlighted by the red overlay in sample (a). 
10

Figure 5: Residual stress results from cut compliance testing. The cut progresses 

from the top surface (on left at 0.00), through the matrix Ti-6Al-4V material (up to 

0.22 inch), and then into the Ti-6Al-4V–SMA composite area (denoted by red shaded 

area).
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Bi-Metallic Composite Structures With Designed Internal Residual

Stress Field 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Brice, Craig A.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20364

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 26

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

Shape memory alloys (SMA) have a unique ability to recover small amounts of plastic strain through a temperature induced 

phase change. For these materials, mechanical displacement can be accomplished by heating the structure to induce a phase 

change, through which some of the plastic strain previously introduced to the structure can be reversed. This paper introduces 

a concept whereby an SMA phase is incorporated into a conventional alloy matrix in a co-continuous reticulated arrangement 

forming a bi-metallic composite structure. Through memory activation of the mechanically constrained SMA phase, a 

controlled residual stress field is developed in the interior of the structure. The presented experimental data show that the 

memory activation of the SMA composite component significantly changes the residual stress distribution in the overall 

structure. Designing the structural arrangement of the two phases to produce a controlled residual stress field could be used to 

create structures that have much improved durability and damage tolerance properties. 

15. SUBJECT TERMS

additive manufacturing; residual stress; shape memory alloy

18. NUMBER

 OF 

 PAGES

15

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

 694478.02.93.02.12.36.23 

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2014-218174

16. SECURITY CLASSIFICATION OF:

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and 

Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person 

shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 02 - 2014